The present invention relates to a code error detecting system capable of detecting a line fault of a relay section in a wireless digital relay transmission system using a digital code phase-modulation system.
In the wireless transmission of digital signals, it is often the practice to cause the digital signals to 4-phase phase-modulate a radio frequency carrier wave for transmission. The transmitted carrier wave is demodulated by coherent detection on the receiver side. When the carrier wave is in the millimeter or the quasi-millimeter wave region, the channel capacity per carrier wave frequency is much greater than in the conventional microwave transmission system. If a carrier wave of one frequency is driven out of order, therefore, a great number of communication channels are affected all at the same time. In addition, in a long haul relay transmission system, it is always a time-consuming process to locate the affected one among many relay sections. One prior art method of the fault detection and location has employed a repetition of a simple code pattern represented by the phase modulation of carrier wave which is transmitted every time the transmission line is out of order. For example, the code-representing sequence of phases O, .pi., O, .pi. . . . , or O, .pi./2, .pi., 3.pi./2, O, .pi./2 . . . are transmitted in place of the information signal. The code pattern is detected to enable the error detection at each relay section by utilizing self-correlation. However, when the 4-phase phase-modulator to be employed for the transmission of the information signal at the transmitter side is composed of a series combination of a .pi./2 phase-modulator and a .pi. phase-modulator in each relay section, a line fault ascribed to a fault with the .pi./2 phase-modulator cannot be detected during the transmission of the phase-modulated carrier wave taking the phase values O, .pi., O, .pi. . . . because this phase-shift modulation is not in phase with of the .pi./2 phase-modulator. While, when the phase-modulated carrier wave successively takes the phase values O, .pi./2, .pi., 3.pi./2, O, .pi./2 . . . representative of the fault-detection code pattern, a fault with the .pi./2 phase-modulator can be detected. In this operation, the change in the phase of the phase-modulated carrier wave is in either the forward or the backward direction, with the repetition rate being set as short as 4 bits. The phase-modulated carrier wave has a spectrum of frequency components distributed by a frequency deviation of Nfc/4, where fc stand for a clock frequency; and N, a positive integer. Consequently, when such a carrier wave received at the receiver side is demodulated, a DC component is inevitably present in the phase-detected output of a carrier wave reproducer circuit employed in the phase-demodulator, so that the reproduction of the carrier wave for phase demodulation is disturbed. This gives rise to the so-called pseudo lock-in phenomenon in which the carrier wave reproducer circuit stays synchronized at a frequency deviated from the input carrier wave frequency.